Accessing Class Members of an Argument within a Member Function

Question

I'm writing up some code as part of a little project I'm working on, and something stood out to me as I was testing my code. I'm working with a class function shown below:

class Matrix{
    public:
        //Constructors
        Matrix();                   //EMPTY Matrix
        Matrix(int, int);           //Matrix WITH ROW AND COL
        Matrix(const Matrix &);     //Copy a matrix to this one
        ~Matrix();                  //Destructor

        //Matrix operations involving the matrix in question
        Matrix madd(Matrix const &m) const; // Matrix addition

    private:
        double **array; //vector<vector<double>> array;
        int nrows;
        int ncols;
        int ncell;
};

Below, paying attention to the madd function, I've written it out in another file shown below:

Matrix Matrix::madd(Matrix const &m) const{
    assert(nrows==m.nrows && ncols==m.ncols);

    Matrix result(nrows,ncols);
    int i,j;

    for (i=0 ; i<nrows ; i++)
        for (j=0 ; j<ncols ; j++)
            result.array[i][j] = array[i][j] + m.array[i][j];

    return result;
}

I suppose you can guess that it does matrix addition. To be honest, I found some code online, and I'm just trying to modify it for my own use, so this function wasn't completely written by me. I managed to compile this, and after a small test, it worked correctly, but what I'm confused by is how in the function I was able to call m.ncols and m.nrows. Looking at the class definition, the members are private, so how am I allowed to access them? Even though the argument m is passed as a const, since the nrows and ncols parameters are protected, shouldn't I NOT be able to access them (from the argument)? I'm confused as to why this is allowed.

fiddling with some code that you found online will not help you learn the language, grab [a good c++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — skeller, May 30 '19 at 19:29
public = everyone can access; protected = class and derived can access; private = access only within same class — skeller, May 30 '19 at 19:32

score 3 · Accepted Answer · answered May 30 '19 at 19:33

3

From access specifiers at cppreference.com

A private member of a class can only be accessed by the members and friends of that class, regardless of whether the members are on the same or different instances:

class S {
private:
    int n; // S::n is private
public:
    S() : n(10) {} // this->n is accessible in S::S
    S(const S& other) : n(other.n) {} // other.n is accessible in S::S
};

answered May 30 '19 at 19:33

Amadeus

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Thanks for the reply. It's been a while since I've actually worked with classes, so this is a nice refresher for me. Thanks for your answer. – BestQualityVacuum May 30 '19 at 19:41

Accessing Class Members of an Argument within a Member Function

1 Answers1